ANS:A - Lewis number

The Lewis number plays an important role in problems of simultaneous heat and mass transfer. Explanation:

1. Lewis Number (Le):
   • The Lewis number is a dimensionless parameter that characterizes the relative importance of thermal diffusivity to mass diffusivity in a fluid.
   • Mathematically, it is defined as the ratio of thermal diffusivity to mass diffusivity: 𝐿𝑒=Thermal DiffusivityMass DiffusivityLe=Mass DiffusivityThermal Diffusivity​.
   • The Lewis number quantifies how quickly heat can be conducted compared to how quickly mass can be transported through a fluid medium.
   • In problems involving simultaneous heat and mass transfer, such as in convective heat transfer with mass transfer (e.g., evaporation, condensation), the Lewis number governs the relative rates of heat and mass transfer.
   • It influences the temperature and concentration profiles near the interface between phases and affects the overall heat and mass transfer rates.
2. Other Parameters:
   • The Schmidt number (Sc) characterizes the relative importance of momentum diffusivity to mass diffusivity in a fluid and is not directly related to simultaneous heat and mass transfer.
   • The Prandtl number (Pr) characterizes the relative importance of momentum diffusivity to thermal diffusivity and is primarily relevant in problems involving convective heat transfer without mass transfer.
   • The Sherwood number (Sh) characterizes the mass transfer rate in a fluid flow and is relevant in problems of mass transfer, but it does not explicitly consider simultaneous heat transfer.